How to choose an electrical cable size

Quick answer: Establish the design load current, then check the cable’s current-carrying capacity under its actual installation conditions, voltage drop and circuit protection. Conductor material, cable construction and terminations must also suit the application. Appliance wattage or the existing breaker rating alone cannot determine the cable size.

This guide forms part of the cables, sockets and connectors section. The steps below explain what information to gather and which checks to complete before selecting a conductor size.

1. Establish the load current

For a single-phase resistive load, approximate current is I = P / U. A 3,000 W heater operating at 230 V therefore draws about 13 A. See volts, amperes and watts for the units and converting watts to amperes for worked examples.

Do not insert a motor’s mechanical output power directly into that formula. Use its nameplate current, supply arrangement and duty. Single-phase and three-phase supplies can produce different line currents for the same power. Motor starting current also needs consideration when assessing starting method, voltage drop and protection.

2. Choose the conductor material and cable construction

Equal conductor sizes do not make cables interchangeable. Copper and aluminium conductors require different assessments. The guide to NYA, NYM, TTR and Antigron cables explains construction differences and designations commonly used in Turkish installations. For exposed or buried routes, check outdoor and underground cable suitability separately.

Conductor construction affects terminal selection; see solid and stranded conductors. If a specification uses AWG, the AWG to mm² conversion guide helps identify conductor area. The conversion does not establish an allowable current rating.

3. Apply current ratings to the actual installation

When using a cable current-carrying capacity table, select the correct conductor material, insulation, number of loaded cores and installation method. Conduit, cable tray, thermal insulation and buried routes present different conditions. Apply the required temperature and grouping correction factors.

Illustrative example: If the appropriate reference rating is 25 A and the applicable correction factors multiply to 0.80, the corrected capacity is 25 × 0.80 = 20 A. These figures demonstrate the method; they are not a universal rating for a particular conductor size. The guide to 1.5, 2.5, 4 and 6 mm² cable applications provides common use cases.

4. Check cable length and voltage drop

A cable that passes the current-carrying check may still need a larger size to limit voltage drop over a long run. Read voltage drop in long cables for the underlying effect and voltage-drop calculations for the methods. Record the actual one-way route length from the supply point to the load.

For a simplified copper circuit supplying a single-phase resistive load, ΔU ≈ 2 × ρ × L × I / S. L is the one-way length and S is conductor area; the factor of 2 includes outgoing and return conductors. Taking ρ ≈ 0.0175 Ω·mm²/m at 20 °C, a 40 m run carrying 10 A through 2.5 mm² conductors gives about 5.6 V drop, or approximately 2.4% of 230 V. Operating temperature, power factor and, where relevant, reactance change the actual result. Check the permitted drop against the project requirements.

5. Coordinate cable sizing with circuit protection

A basic overload coordination condition is I_B ≤ I_n ≤ I_Z. I_B is design load current, I_n is the protective device’s rating or setting, and I_Z is the cable’s corrected capacity. This does not complete the protection assessment: operating characteristics, breaking capacity, cable short-circuit withstand and required disconnection time also need verification.

Type B, C and D circuit breakers have different instantaneous tripping characteristics, even at the same current rating. MCBs, RCDs and RCBOs serve different functions; an RCD without overcurrent protection does not protect a cable against overload. Fitting a higher-rated breaker without assessing the circuit is not a remedy for tripping.

6. Check the neutral, protective conductor and terminations

L, N and PE markings identify conductor functions. Neutral and protective conductor sizes need their own assessment under the applicable circuit conditions and rules. Selecting the phase conductor does not complete those checks. The earthing arrangement must also be considered alongside fault protection.

Terminals, ferrules and cable lugs must accept the chosen conductor size and class. Connectors and cable joints also affect circuit reliability. A socket-fed load needs a separate check of socket capacity. A larger cable does not increase the rating of an underspecified connector.

7. Confirm the final size against every requirement

The selected size must satisfy load current, corrected current-carrying capacity, voltage drop, short-circuit withstand and protection requirements together. It must also meet equipment connection limits and the applicable installation rules. A circuit operating normally is not proof of compliance; cable overheating can result from excessive loading or a poor termination.

Application note: This guide explains the sizing process. Mains circuit design, installation and final verification require competent personnel. Stop using a circuit if you notice burning smells, melting or excessive overheating. Isolate the supply if you can do so safely and arrange an electrician’s inspection.